From hitherto, fading and discoloration of colored decorative coating films and the like formed on any of various substrates (e.g. printed articles, building materials, fibers, organic polymer resin products, etc.) using organic dyes and pigments has been a problem. Factors in such color degradation include photooxidation, photoreduction and thermal reactions, and various methods have been thought of as countermeasures.
For example, to prevent degradation of an organic dye or pigment or the surface of an organic polymer resin sheet, methods have been adopted such as mixing into the material an ultraviolet absorber that absorbs energy that will bring about photooxidation, or forming an oxidation-preventing film on the surface of the substrate.
Moreover, removing soiling such as oil on the surface of any of various substrates comprising either an organic or inorganic material requires effort, and hence methods of forming a coating film having an anti-soiling function or a self-cleaning function have been developed. As such a method, for example substrates that make use of a photocatalytic function using anatase-type titanium oxide developed by the present inventors have received attention.
However, an aqueous liquid or dispersion containing anatase-type titanium oxide developed by the present inventors contains not only titanium peroxide having a photocatalytic function, but also any of various metal compounds of copper, tin, iron, zinc, indium, silver, calcium, aluminum, nickel, silicon, selenium or the like, and hence in the case of using such an aqueous liquid or dispersion with an organic material such as a resin or an organic dye, degradation such as discoloration or fading through sunlight or the like has been unavoidable.
Moreover, as a film formation method that aims only for a substrate surface self-cleaning effect, art is known in which a film of silica or a silica compound is formed on the substrate, thus making the substrate surface hydrophilic so that soiling can be removed using running water.
However, a surface on which a film of anatase-type titanium oxide or the like having a photocatalytic function has been formed has strong adsorptivity, and hence adsorbs substances that are to be subjected to soiling decomposition, and then these substances are decomposed by the excitation wavelength of sunlight or the like. Application of such film formation to an inorganic substrate is relatively easy, but with application to an organic substrate, degradation is brought about through decomposition of the organic substrate surface, and hence this has been coped with by forming a primer as a first layer and then a photocatalytic film as a second layer. Consequently, it has been impossible to attain both prevention of degradation caused by photooxidation of the organic substrate and an anti-soiling function using photocatalysis, with these inherently running counter to one another.